1. Field of the Invention
This invention relates to the roasting of sulphide materials, that is sulphide ores or concentrates, and more particularly to the roasting of zinc/lead sulphide materials which contain more than about 5% by weight of lead sulphide.
2. Description of the Prior Art
The normal practice, when roasting mixed zinc and lead sulphide concentrates prior to smelting in a zinc/lead blast furnace, is to sinter these on a Dwight-Lloyd sinter machine with upflow of air to support combustion of the sulphides. Using this technique concentrates containing up to about 25% by weight of lead suliphide can be handled.
There is, however, a need to develop a roasting process for mixed zinc/lead sulphide concentrates which is independent of this sintering technique. It is known that if such materials can be calcined to particulate oxidic form then they can be densified into briquettes, suitable for feeding to a blast furnace, e.g. by the techniques described in our British Pat. Nos. 1 302 864 and 1 394 609.
In the electrolytic winning of zinc it has become standard practice to roast zinc sulphide concentrates in a fluidized bed roaster (see for example the process described in British Pat. No. 715 167). The fluidized bed roasting process has the advantages of virtually complete sulphur elimination from the solids, relatively easy heat recovery from the gas, and good gas/solid mixing in the bed. However, the fluidized bed roasting technique will not operate satisfactorily when the solid feed to the fluidized bed contains more than about 5% by weight of lead sulphide. This is because the solids become increasingly sticky with increasing lead content, leading to agglomeration of the solids and a breakdown of fluidization. The molten phase leading to this stickiness in the bed is believed to be composed of a lead oxide/lead sulphate phase which is molten above about 750.degree. C.